Electrical contact



Patented June 29, 1954 TENT F ICE ELECTRICAL CONTACT Alfred L. Jenny,Pittsfield, Mass., assignor to General" Electric .Company, acorporation. of

New York No *Drawing. Application J une*28, 1949, l

Serial No 101,910

4 Claims. (Cl. 200-166) The present invention'relates to an electricalcontact. It is more particularly'concerned with a switch contact whichis resistant to corrosion by sulfur when immersed in oil and to a methodof making such contact.

Silver is one of themost widely used metals for electrical contactpurposes. However, silver contacts are not "wholly satisfactory for oilim-' mersed switching apparatus due .to-the forma-'= tion of silversulfide scale on the exposed'or nonengaging areas of the'contactsurface. As this sulfide is a good conductor of electricity, it islikely to cause electricalfailures when'it drops oifthe contact surfaceandcollects on insulated or other parts of the oil immersed switchorother electrical equipment. It is important, therefore,

to prevent sulfide formation on all parts of solid silver or silverplate applied for contact purposes and a primary object of the presentinvention is to provide such an electrical contactwhich is realloys ofcopper and silver can be used if desired,

the only requirement being that the exposed surface of the electricalcontact be composed of a layer of silver amalgam, that is, asilver-mercury alloy, preferably containing from about 5 to percentmercury.

As copper is the most commonly used conductor, the invention will beparticularly described,

with reference to a switch contact comprising a copper base or backing.Employing such a base, the copper part is first provided with a coatingof silver. The silver suitably may be applied by electroplatingemploying anysuitable. silver plating bath and allowing the part toremain in the bath until a silver layer of the desired thick ness,usually from about to 5 mils, has been obtained.

The surface layer of silver is next alloyed with mercury to provide asilver amalgam layer of about thersame. thickness containing from 5 to25 percent by weight of mercury. The mercury preferablyis applied byplating onto thesilvenv surfaced part in order to obtain a uniform"dis=tribution of the mercury and is thereafter causedto diffuse into andalloy with the-silver. natively, the silver-mercury layer may be formedAlter :sponding iodides.

in itself is more resistant to sulfiding than silver by codeposition of:silver and mercury on the metal base.

The invention is not limited to the use of any particular mercuryplating bath. The mercury is electro-deposited on the silver'surface inan amount sufficient to provide a silver-amalgam layer of the desiredcomposition. When first plated, the mercury surface has a high lustrewith the characteristicappearance of mercury. To obtain the alloyformation the mercury plated part is aged at room or elevatedtemperature during which time the lustre disappears and is replaced by adulLmatte surface not unlike the original silver surface. The requiredamount of diffusion is usually accomplished byimmersion of the part inboiling water for from 2 to minutes, generally about 15.minutes. Agingat room temperature for about one weekproduces about the same degree ofalloying .as 15 minutes at C.

When the proper silver-mercury layer has been developed, the surfacethereof is passivated by a suitable .treatment designed -to convert thesilver and mercury at the surface to the corre-- Although the silveramalgam alone; after longi'periods J of timeithe mercury graduallydiffuses through-the entire piece leaving practically free silver on thesurface. The immediate formation of an iodide film in accordance withthe present invention" captures the proper amounts of silver and:mercury on the surface as iodides, passivating the surface, and removingany concern about the subsequent loss K of the remaining mercurybyfurther diffusion.

iodization is preferably accomplished 'anodically in a'suitable'alkalimetaliodide solution such as a 10 percent solutionof potassium iodidehaving a pH- of about 10.- As it is desired to convert as much of themercury aspossible to mercurous iodide,-the anodic treatment should becarried out at a current density less than 15 amperes per square footand preferably less than 10 am peres per square foot with the time oftreatment being such that the mercury iodide comprises at least 5'percent of the metal iodide content of the surface film. It hasbeenfound that the ratio of HgI to AgI in the film varies inversely as thecurrent density and time of treatment. While this ratio also variesdirectly as the mercury content of the amalgam, it is usually higherthan the Hg/Ag ratio of the alloy particularly at low current densitiesand short treating times and films having very good sulfide resistancehave been obtained containing as high as 8085% HgI, balancesubstantially AgI.

The effect of time and current density on the composition of the iodidefilm are shown by the results of a series of tests in which coppercontacts surfaced with a layer of 90 percent silverpercent mercuryobtained by diffusion for minutes in boiling water were anodicallytreated in a 10 percent potassium iodide solution as indicated in TableI.

TABLE I Treatment igg Sample (mun) (Amps'l'sq' l6 3. 18. 6 17 and 18 59. 2

Each sample contact was weighed after treatment to determine iodinepickup and the film weight determined by rapid solution in sodiumcyanide. The composition of the films, i. e., the relative proportion ofsilver iodide and mercurous iodide was calculated by dividing the filmWeight by the iodine pickup to obtain a factor. This factor for puresilver iodide is El I or 1.85 and for pure mercurous iodide is ll l I or2.58. On the proper assumption that the films contain only iodides, anymixture of silver and mercury iodide has a factor between the two above,and is indicative of film composition by the inverse lever law.

The composition of the iodide films obtained on the above-describedsamples are set forth in Table II.

TABLE II Per- Perm ms. Milligrams Film s Samples Factor cent cent HgI Iplckup Agl HgI formed 1 and 21 29. 85 62. 85 2. 1 65 22. 0 3 and 4- l48. 85 98. 8 2. 02 77 23 22. 8 5 and. 6- 68. 85 136 1. 975 83 17 23. 0 7and 8 31.05 61.25 1.975 83 17 10. 5 9 and 10..- 65. 7 126.1 1.92 90. 59. 5 12.1 11 and12.. 103. 6 197. 1.90 93 7.0 13. 8 13 30 58,6 1.95 86 148.2 1 1 23. 2 49. 4 2. 12 63 37 18. 2 15and16. 123 220 1.79 100 0.0 0.017andl8 68.7 130.5 1.9 93 7 9 From the data of Tables I and II, it isapparent that the composition of the iodide film is a function of (a)the current density and (b) the time of treatment and can be controlledby current density or time of treatment or both. The results obtainedwith samples 15 and 16 show that no I-IgI can be obtained at currentdensities of 15 amperes per square foot or higher even when the silveramalgam surface contains a substantial amount of mercury.

The high resistance to sulfide formation of two iodized silver amalgamsas compared with an untreated silver surface and an iodized silversurface and the desirability of having an iodide film comparatively richin mercurous iodide is shown by a series of tests on sample contactstreated as shown in Table III.

Samples 1 and 2 were pure silver blanks, 3 and 4 were treated to have arelatively HgI rich film, 5 and 6 were treated to have a relatively AgIrich film, and '7 and 8 were 100% Ag]: films. Each sample was exposed to125 cc. of oil containing 30 milligrams available sulfur (.03% S), andthe test continued for 980 hours at 60 C. After 500 hours, portions ofthe silver sulfide growing on the surface of the AgI film (7 and 8) weredislodged in degreasing, handling and weighing these samples so thefurther tests on these samples were meaningless. The increase in weightof the various sample contacts as a result of sulfide formation that isthe weight of sulfur picked up out of a total of 30 mg. availablesulfur, are given in Table IV.

TABLE IV Sulfur pickup (mg.) after exposure time (hours) samples m 33.35?. its. $2.

1 and 2 (ave.) "mg" 28. 4 28.8 29. 3 29. 3 29. 3 d 4 0.0 0.0 0. 25 1. 61o. 0 0.85 3. 0 3.80 3. 75 12. 6 1. 45 2.85 24. 2 15. 20 14. s

From these results it is seen that the iodide films on the silvermercury alloys afford excellent protection against sulfiding types ofcorrosion even under the highly accelerated conditions of the test. Itis also evident that the best protection is noted with the iodide filmscomparatively rich in mercurous iodide so that accordingly, it isdesirable to employ contacts in which the iodide films comprise at least15 percent mercurous iodide.

The contact resistances of the iodide coated silver-amalgam contacts arecomparatively low as shown by the following test results on variousoil-immersed cooperating in. rounded contacts operating at 80 C. and 20pound contact pressure and a current of amperes A. C.

As compared with the straight silver iodide coated contacts, thecontacts of the present invention have surfaces which are much lessporous or more dense probably due to the mercury impregnating the silversurface or to the mercury iodide formed upon iodization.

While the invention has been specifically described with reference tothe use of a potassium iodide solution for forming the iodide surfacefilm, it is of course not limited thereto. For example, any suitableiodide such as a solution of sodium iodide may be substituted for thepotassium iodide. Alternatively, while a longer exposure time will berequired, satisfactory iodide films rich in HgI can also be formedmerely by immersion of the silver amalgam contact in a solution ofiodine, for example, in carbon tetrachloride.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A switch contact comprising a high conductivity metal base, and acoatin of a silverrich silver-mercury alloy containing from 5 to 25 percent mercury the surface of which is composed of silver and mercuryiodide, the mercury iodide comprising at least 5 percent by weight ofthe combined iodide components of the surface.

2. An oil-immersed switch contact havin a coating of silver containingfrom 5 to 25 percent mercury, the silver and mercury at the surface ofsaid coating being substantially in the form of silver iodide andmercury iodide.

3. An oil-immersed switch contact having a coating of a silver amalgamcontaining about percent mercury, the surface of said coating comprisingessentially silver iodide and mercury iodide with the mercury iodidecomprising about 35 percent of the iodide content of the surface.

4. The method of making an electrical contact resistant to corrosion bysulfur which comprises providing a copper base with a layer of silverhaving a thickness of from about to 5 mils, electroplating a layer ofmercury 0n the silver layer such that on difiusion of the mercury intothe silver layer there is obtained a silver-mercury layer containingabout 10 percent mercury, making the resultant product the anode in adilute aqueous solution of an alkali metal iodide and passing currentthrough said solution and anode of a current density less than 10amps/sq. ft. for a time sufiicient to form on the surface of thesilver-mercury layer a film comprisin silver iodide and mercury iodidein which the mercurous iodide comprises from 5 to percent of the totalamount of iodides in said film.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 26,978 Edwards Jan. 31, 1860 1,602,595 Sheppard et al. Oct.12, 1926 1,720,216 Gray et al. July 9, 1926 1,758,293 Murray May 13,1930 1,860,505 Jones May 31, 1932 2,116,215 Ruben May'3, 1938 2,195,231Weder Mar. 26, 1940 2,281,446 Laise Apr. 28, 1942 2,430,468 Julich etal. Nov. 11, 1947 2,469,878 Hannon et al May 10, 1949 2,583,581 LukensJan, 9, 1952 OTHER REFERENCES Silver in Industry, by Addicks (1940),page 310.

1. A SWITCH CONTACT COMPRISING A HIGH CONDUCTIVITY METAL BASE, AND ACOATING OF A SILVERRICH SILVER-MERCURY ALLOY CONTAINING FROM 5 TO 25 PERCENT MERCURY THE SURFACE OF WHICH IS COMPOSED OF SILVER AND MERCURYIODIDE, THE MERCURY IODIDE COMPRISING AT LEAST 5 PERCENT BY WEIGHT OFTHE COMBINED IODIDE COMPONENTS OF THE SURFACE.